An optical system and a method for transmitting a source image are disclosed in the article by Tapani Levola: “Diffractive optics for virtual reality displays”, Journal of the SID 14/5, 2006, pages 467 to 475. Further optical systems and methods for transmitting a source image are known from US 2014/0140653 A1, U.S. Pat. No. 8,233,204 B1, US 2014/0218801 A1, which disclose an optical system according to the preamble of patent claim 1. A further optical system for transmitting a source image is known from US 2006/0221448 A1.
An optical system of this type and a method of this type are used for example in a display system, such as an HUD (head-up display) or HMD (head-mounted display). An HUD and an HMD are display systems in which a source image is projected into the user's field of view with magnification of the exit pupil. In this case, the source image can be for example the image of a display of a vehicle instrument, of a cellular phone, of a games console, of a computer and the like. HUDs are used nowadays for example in aircraft and motor vehicles in order to project information, for example navigation information and the like, for the pilot or driver into the latter's field of view, without the pilot or driver having to divert his/her viewing direction from the straight-ahead viewing direction. An HMD, in contrast to an HUD, is worn on the user's head. An HMD either presents images on a screen close to the eyes, or projects the images directly onto the retina. Other designations for an HMD include video glasses or smartglasses, helmet display or virtual reality helmet.
The principal components of such display systems are a display unit, which supplies the source image from a connected data source, and an optical system for transmitting the source image into a target image.
One important characteristic variable of such display systems is the field of view (FOV). The field of view of such display systems desirably has a magnitude such that the entire source image is transmitted into the target image. The field of view is the difference between the maximum and minimum angles, in each case measured from the center of the image to the mutually opposite image edges in a horizontal dimension (horizontal field of view) and in a vertical dimension (vertical field of view). In the present description, reference is made only to the field of view in one dimension.
The optical system of such display systems, as is disclosed in the article cited above, have as component parts an optical waveguide arrangement having one or more optical waveguides, in which optical waveguide arrangement light can propagate by total internal reflection at optical interfaces, a diffractive optical input coupling arrangement, by which the light emanating from the source image can be coupled into the optical waveguide arrangement, and a diffractive optical output coupling arrangement, by which the light that has propagated in the optical waveguide arrangement can be coupled out from the optical waveguide arrangement, such that the light can enter one or both of the user's eyes. In this case, the optical waveguide arrangement can have one or more optical waveguides, and the input coupling arrangement and the output coupling arrangement can have one or more diffraction gratings.
In general, in the case of optical systems having the construction described above, it has been found that the field of view of such an optical system is restricted, that is to say that the entire source image or, in other words, the entire field angle spectrum of the light emanating from the source image cannot be transmitted by the optical system. In the case of relatively large source images, for example in the 16:9 format that is customary nowadays, edge regions may be absent in the transmitted image.
Generally, the field of view is small in the case of optical systems having the construction described above. In the case of HMDs, in particular, there is by contrast the desire for the largest possible field of view with image angles of the field of view of more than 20°, and preferably more than 40°.
During the transmission of polychromatic source images, as is the case for example during the transmission of video images, encompassing the entire visible spectrum in the wavelength range of approximately 425 nm to approximately 675 mm, the further issue can arise that the field of view becomes all the smaller, the larger the spectral range to be transmitted. In general, the field of view is restricted, however, even during monochromatic transmission.
During polychromatic transmission, on account of the wavelength dependence of the diffraction, this can additionally have the effect that the transmitted source image does not have color fidelity relative to the source image to be transmitted because for example the entire wavelength spectrum of the source image is not transmitted into the target image or different spectral ranges having different intensities are transmitted.
A further property of such an optical system that can restrict the field of view involves propagation of the light in the optical waveguide arrangement by total internal reflection. This type of light propagation is present, however, if the optical system is intended to be transparent at least in the user's field of view, as is desired in the case of an HUD or HMD, in particular smartglasses, such that the user can see the transmitted source image in superimposition with the real world. Generally, total internal reflections within the optical waveguide arrangement occur, however, only if the light incident on the optical interface(s) of the optical waveguide arrangement has an angle of incidence that is greater than the critical angle of total internal reflection.